You probably won't need to go back to get a second Bachelor's--but you may need to go back to school. What you need to do depends on what you want to do with your geology degree. If you want to teach earth sciences in a middle or high school setting, you don't need a new bachelor's degree--you'll just have to go through the certification process and and pass the Praxis II test for earth science.

If you want to pursue a career as a geoscientist, you'll probably need a Master's or Ph.D in geology, even for entry-level positions. You'll need a Bachelor's degree to get into these programs, but you may not need a Bachelor's in geology specifically. You'll need to do well on your GRE's, particularly in the sciences, and justify in your personal statement why you want to go into geology.

The U.S. Department of Labor has a lot of information on geoscience and its different specialties, as well as the requirements you need to get a start in the field. You can check it out here: